In the manufacture of continuous glass fibers, glass batch ingredients are melted in furnaces. The molten glass resulting from the melting of the glass batch mixtures is then fed into elongated forehearths of various sizes and shapes. The molten glass flowing in the forehearths is removed at many locations positioned along the length of the forehearths from fiber glass forming bushings or spinnerettes. In a typical fiber glass manufacturing operation, a glass level of predetermined height is maintained in the forehearth being fed from the glass furnace. These forehearths are normally constructed of ceramic material resistant to molten glass attack and are frequently heated by combustion gases to maintain the molten glass during its passage through the forehearth at a given temperature. In some instances, electrodes can be used to supply electrical energy to the forehearth to maintain the glass at a desired temperature.
The bushings utilized to form the glass fibers are typically precious metal containers. These bushings are provided with a plurality of orifices usually on the bottom thereof through which molten glass housed in the container may pass freely to exit the bushing and form fibers. The orifices are arranged usually in rows and vary in diameter and number from bushing to bushing depending on the product strand being made from a given bushing. Bushings are typically mounted in cast iron frames and the frame has within its confines a castable ceramic surrounding the bushing metal to electrically isolate the bushing from the frame material. During operation the bushing is electrically heated to maintain a given uniform temperature therein. The bushing, through its frame, is bolted to a bushing block which is in fluid communication with a melting furnace forehearth. U.S. Pat. No. 3,837,823 gives a general description of the mounting of a bushing in a bushing frame and a bushing block. This patent also shows, in general, an arrangement of fiber glass furnace or melter and its associated forehearth and bushing positions.
It is common for a given furnace feeding a forehearth to have positioned in that forehearth as many as 40 to 100 bushings. These bushings are subject to frequent replacement due to change in demands for products being made in the market place as well as for repairs caused by mechanical failures such as leaks or cracks occurring in the bushings themselves. When a bushing must be changed to accommodate the changing requirements of the marketplace, for example, by replacing a bushing manufacturing one kind of strand with a bushing manufacturing another, or when a bushing must be replaced for repair caused by a leak or crack, present procedures require considerable time and effort.
In order to remove a bushing from its position on the forehearth, the bushing first has to have its electrical supply cut off. Once this is done, the glass contained in the bushing itself is allowed to cool until it solidifies. The bushing and the associated ceramic blocks located above it must also be chilled in order to insure that molten glass in the bushing block above the bushing per se has solidified prior to the removal of the bushing from its connection to the block. When glass in the block is solidified the bushing may then be chiseled away from the block. The chilling of the bushing block and the bushing is usually carried out by directing water onto the surfaces of the bushing for considerable periods of time. The chiseling of the bushing to separate it from its block and the chilling of the bushing and its associated bushing blocks introduce serious problems in the glass forming area.
First the chilling causes a thermal shock to glass flowing in the forehearth in the vicinity of the bushing position being removed. The chiseling of the glass, on the other hand, at the bushing block interface causes physical shock to the forehearth refractories in the vicinity of that position. These shocks cause thermal disturbances as well as contamination of the glass flowing in the forehearth which has upsetting effects on adjacent bushing positions within the forehearth. In addition, the shock cooling such large masses of glass and the subsequent mechanical operations which introduce physical as well as thermal shock to the forehearth are time consuming in addition to being deleterious. As will be apparent, they also reduce efficiency and the productivity of the entire forehearth area.
Thus, a need persists in the art to provide it with a rapid and easy method for the removal of bushings from forehearths with a minimum of disturbance of the thermal and physical environment surrounding the bushing being removed. The method also should be fast and efficient so that the loss of productivity during changeover of a bushing can be minimized.
One patent which has directed itself to this particular need is U.S. Pat. No. 4,525,188. In this patent, gas pressures are utilized to maintain glass flowing in a bushing below the level of the bushing flange while maintaining it above the level of the glass inlet to the bushing. Glass inlets are supplied which are of a small cross sectional area. This system provides a method of removing bushings which involves pumping gas into a bushing to push the glass contained within the bushing below the level of the gas inlets in the bushing while simultaneously disconnecting power to the bushing to thereby freeze off the reduced cross-sectional area inlets to solidify the glass therein and permit the bushing to be removed. While this process can be used efficaciously in providing for a rapid removal of a bushing, it does require careful control of glass levels within the bushing itself and delicate instrumentation to determine the glass level. It also requires the constant manipulation of gas pressures to maintain glass levels in bushings constant and to control glass levels when it is desired to remove the bushing.